/mail is a useful command if you want to message someone something when they're not online. But, you have to type the message in along with the command in the text box, and unfortunately, it has a character limit that restricts you from sending a long message, so if you want to send something that overrides the character limit, you have to spread it across multiple messages.
What I'd really like to see is a revamp of the /mail command. Rather than typing "/mail send [username] [message], why not have it function somewhat similar to /gift? Example would be where you do /mail [username], and a book appears. You write in that book, and as soon as you've signed the book, it'll automatically get sent to the receiver. The receiver would get a notification when the /mail has been sent (or when he/she logs in if the /mail was sent when he/she was offline), and does /mail read, where he/she'd get the book with the /mail message in it. Once the receiver has read the message in the book, he/she does /mail clear and all of the /mail books that are in her inventory despawns.
This would prove practical because it would allow for (VERY) long messages to be sent in a single go, rather than having to spread it out with multiple /mail messages due to the character limit, as I stated earlier.